We are referring to tank additives other than saltwater. The most common being some form of a calcium/buffer supplement, like B-Ionic. BTW, don't add anything to the tank that you haven't tested for. Many LFS stores will gladly sell iodine, iron, magnesium, ich cure, magic reef in a bottle and countless other unecessary additives to the beginner. Don't fall for it. You will likely have to have some form of calcium and alkalinity additive, depending largely on the salt you are using. You should have good test kits for both.
I'd recommend ammonia, nitrite, nitrate, phosphate, ph, alkalinity and calcium. I like Salifert kits myself. If the tank is established you can get by with just calcium and alkalinity kits but the others are all good to have on hand for troubleshooting problems. Obviously you also need a refractometer or hydrometer for checking specific gravity.